Much research and development effort has been devoted to perfecting new and better techniques for filtering biological fluids and for separating the constituent parts of such fluids. Especially important are techniques for separating whole blood into its constituent elements ("hemapheresis"). Although most hemapheresis has been performed in the past by centrifuging batches of whole blood, batch processing is very expensive and time-consuming. Batch processing is now being replaced, wherever possible, by more efficient, continuous-flow blood filtration techniques. See, for example, application Ser. No. 449,470, filed Dec. 13, 1982, entitled "Blood Fractionation System and Method"; application Ser. No. 591,925, filed Mar. 21, 1984, entitled "Method and Apparatus for Separation of Matter from Suspension", abandoned,; U.S. Pat. No. 3,519,201, entitled "Seal Means for Blood Separator and the Like" issued in 1970 to Eisel et al; and U.S. Pat. No. 4,303,193 to Latham, Jr. entitled "Apparatus for Separating Blood into Components Thereof".
Continuous-flow blood filtration systems receive a flow of whole blood from a subject, and couple this blood flow to a rotating rotor or spinner rotatably disposed within a fluid-tight housing. The spinning rotor separates the whole blood into its component parts (e.g., in plasmapheresis, the whole blood is separated into packed red blood cells and plasma), and the separated component parts are discharged through different outlets of the housing.
Others have sought to develop reliable pivot bearing arrangements for rotatably supporting the rotor of a fluid filtration system within a housing -- see, for example, U.S. Pat. No. 3,448,858 to Delcellier et al for an example of one such pivot bearing arrangement. Those developing continuous-flow plasmapheresis devices for commercial production have searched for a suitable inexpensive and reliable pivot pin bearing which can rotatably support a separation rotor within a housing and couple fluid flow to/from the rotor -- and also prevent the coupled fluid flow from escaping from the bearing (except through a fluid discharge outlet) while isolating the coupled fluid flow from fluid outside of the bearing.
Sterility is an absolute requirement in continuous-flow type plasmapheresis devices. It is desirable (if not essential) that parts of the plasmapheresis system which are actually exposed to blood flow be disposable so that disease will not be spread from one donor to another. Known sealing pivot pin bearings which reliably rotatably support the separation rotor within a housing and also couple fluid flow to/from the rotating rotor are relatively expensive to manufacture, increasing the cost of disposable continuous-flow plasmapheresis filtration cartridges.
Because the filtration cartridges are discarded after only one use, extended bearing life is not a concern. Only 25-45 minutes of continuous operation is required for plasmapheresis (the filtration cartridge should have a life of 2 hours if used for plateletapheresis, a life of 4 hours if used for certain therapeutic procedures, and a life of up to 6 hours if used for autotransfusion). The pivot pin bearing must perform absolutely reliably throughout its expected life, since premature excessive wear can make the extracted blood unusable and/or interrupt the extraction and filtration process. The sealing pivot pin bearings used in filtration cartridges must also be sterilizable (e.g., by exposure to gamma radiation), and cannot damage the biological fluid flowing through them.
The assignee of the subject application has searched for years for a reliable, inexpensive bearing structure meeting the above requirements for use in its continuous-flow blood filtration system (of the type disclosed in, for example, U.S. Patent Application Ser. No. 591,925 of Schoendorfer). A brief description of that blood filtration system and the stainless steel pivot pin bearings previously used will now be presented in conjunction with FIG. 1 (although a much more detailed discussion of that system may be found in the above-cited, commonly-assigned U.S. patent application).
FIG. 1 is an elevated, perspective view in partial cross-section of a prior art filtration-type plasmapheresis system 10. Plasmapheresis system 10 includes a disposable filtration cartridge 12 and a magnetic driving assembly 14.
Cartridge 12 includes a generally cylindrical, vertically-oriented housing 16 within which an elongated cylindrical rotor ("spinner") 18 is rotatably supported in a vertical orientation between an upper pivot bearing 20 and a lower pivot bearing 22. Rotor 18 rotates within housing 16 about a generally vertical axis 19 of rotation axial to the rotor.
A ring 24 of magnetic material integral to the upper vertical end 26 of rotor 18 is acted upon by a rotating magnetic field generated by external, rotating magnetic drive member 28 (which slides over the housing upper end), causing rotor 18 to rotate relative to housing 16. Magnetic drive member 28 rotates at a predetermined angular velocity (3600 rpm in the preferred embodiment) in response to torque applied to it by a drive motor 30.
Rotor 18 has an internal cavity 31 within it bounded by a grooved cylindrical wall 34. A membrane-type filter 36 covers the outer surface 35 of grooved wall 34. A network of channels 33 formed in the outer wall surface 35 channels fluid trapped between the filter 36 and the outer wall surface into a lower cavity 32 at the rotor lower end 27.
When system 10 is operating, whole blood is delivered to an upper end 37 of housing 16 via a whole blood inlet port 38. The whole blood flows from inlet port 38 into housing 16 and downward into contact with filter 36. Due to the rotation of rotor 18 and the effect of filter 36, whole blood in the space 40 between filter 36 and an inner wall 42 of housing 16 is separated into packed red blood cells (which remain in space 40), and plasma (which flows through filter 36 into a space between grooved wall 34 and the filter). The packed red blood cells continue to flow downward to the lower end 44 of housing 16 and flow out of the housing through a packed blood cell outlet 46. The plasma is channelled by the grooves in rotor wall 34 and channels 33 and flows downward into lower cavity 32.
A lower cap 48 terminating the lower end 44 of housing 16 defines a bore 50 of predetermined length aligned with the axis 19 of rotor rotation. Bore 50 terminates in a circular interior wall (annulus) 52 having a circular orifice 54 defined through its center (terminating interior annulus 52 blocks all but the center of bore 50).
A plasma outlet 56 is defined in lower cap 48 (in communication with bore 50 at the other side of annulus 52). Plasma is discharged from housing 16 through plasma outlet 56. The lower end 58 of lower pivot pin 22 (the pivot pin being vertically oriented in the embodiment shown) is press-fitted into bore 50 and abuts obstructing annulus 52. The upper end 60 of lower pivot pin 22 is rotatably disposed within a bore 62 at the rotor lower end 64 and aligned with the axis 19 of rotor rotation. Rotor 18 spins with respect to lower pivot pin 22 and is supported above and spaced away from housing lower cap 48 by this pin.
Axial bore 62 defined at the rotor lower end 64 terminates in a ridge or annulus 66 having an orifice 68 defined therethrough. Orifice 68 is in fluid communication with rotor cavity 32. A fluid passageway 70 is defined axially through lower pivot pin 22. An O-ring or other resilient seal 72 (Viton elastomer is preferably used) is disposed between annulus 66 and the upper end 60 of lower pivot pin 22. Seal 72 defines a hole centrally therethrough which communicates orifice 68 with passageway 70. Cavity 32 is thus in fluid communication with plasma outlet 56 via orifice 68, the hole through O-ring 72, passageway 70 defined axially through lower pivot pin 22, and orifice 54.
Lower pivot bearing 22 rotatably supports rotor 18 and provides a sealed fluid passageway through which the plasma within rotor cavity 32 can escape. The lower pivot pin upper end is loosely fitted into bore 62 to permit rotor 18 to rotate relative to the pin--the pin being fixed with respect to housing 16 because it is press-fitted into bore 50. O-ring 72 establishes a fluid-tight seal between annulus 66 and the upper end 60 of lower pivot pin 22 which prevents plasma from escaping from cavity 32 except through pin passageway 70. The fluid-tight seal between annulus 66, O-ring 72 and the pin upper end 60 also prevents packed red blood cells collected in space 40 near the housing lower end 44 from flowing upward into bore 62 (between pin 22 and the wall defining the bore) and into contact with the plasma.
The axial downward force exerted on O-ring 72 by rotor 18 insures that a fluid-tight seal exists between annulus 66 of rotor 18, O-ring 72 and the upper end 60 of lower pin 22. This downward force is derived in part from the force of gravity exerted on rotor 18 and in part from the downward component of the rotating magnetic field exerted on the rotor by magnetic drive member 28. Despite this downward force, pin upper end 60 does not wear appreciably over the life of cartridge 12 if the pin is made of hard stainless steel.
Upper pivot pin 20 may be solid (since no fluid passes through it in the embodiment shown), but preferably has a passageway 84 axially defined therethrough so that the upper pivot pin and lower pivot pin 22 can have the same structure--and are thus interchangeable (to reduce manufacturing, inventory and assembly costs). An O-ring and orifices could be provided at the upper end 26 of rotor 18 (as they are provided at the lower end 64 of the rotor) if fluid coupling through upper pin 20 is desired.
As will now be understood, the fluid-tight seal between annulus 66 of rotor 18 and upper end 60 of lower pivot pin 22 is absolutely critical to the proper operation of system 10. The downward axial force applied to O-ring 72 by rotor 18 can cause the O-ring and/or the upper end 60 of pin 22 to wear, in turn causing the fluid-tight seal to degenerate, if the pin is made of improper material. Leakage of this seal can make the plasma 56 discharged from outlet 56 unusable. Moreover, wear of pin upper end 60 can close passageway 70, interrupting the entire filtration operation (to the distress and discomfort of the donor).
The assignee of the subject invention has used precision stainless steel pins 22 and Viton O-rings 72 in the past to provide pivot pin bearings with great wear resistance which overcome problems caused by wear. Unfortunately, precision stainless steel pins are expensive to manufacture and Viton material is also very expensive--substantially increasing the cost of cartridge 12.
The fabrication of these prior art precision stainless steel pivot pins 20, 22 are described in detail in copending commonly-assigned application Ser. No. 722,707. Initial raw material costs for stainless steel pins are relatively high, and machining, polishing, cleaning and other treatment of the stainless steel to produce a finished pin further increases installed cost. Moreover, a large percentage of completed pins are unacceptable for use because of the very close dimensional tolerances necessary to guarantee proper mating of the pins with rotor 18 and housing 16.
Application Ser. No. 722,797 of Schoendorfer and Williamson, now U.S. Pat. No. 4,675,106, discloses a novel injection-molded plastic pivot bearing design suitable for replacing precision stainless steel pivot pins. Such injection-molded pivot pins are formed from a hard plastic having a low coefficient of friction. By choosing an appropriate plastic material for the pin (e.g., nylon 6/6 and RL 4730 polyamide-based polymer modified with PTFE and silicon), the pin has a lubricious wear-resistant characteristic and will not create "hot spots" on relative rotating parts or shed debris. When used in conjunction with an O-ring seal fabricated of a Viton material (a series of fluoroelastomers based on the copolymer of vinylidene fluoride and hexafluropropylene marketed by Dupont), the force applied by rotor 18 on the lower plastic pin and O-ring together provides adequate fluid-tight sealing.
Such a fluid-tight seal lasts long enough to permit the separation and collection of a required amount of plasma. Because cartridge 12 is discarded after one use, self-destruction of the pivot pin bearing due to wear from a few hours of continuous rotation of rotor 18 is of no consequence in blood filtration applications.
Although the pivot pin bearing design described in copending application Ser. No. 722,707, (now U.S. Pat. No. 4,675,106) is successful in its own right and works well for its intended purpose, the Viton seal this design uses is very expensive, and its components must be closely inspected for use in blood filtration applications. It would be highly desirable to provide a low-cost injection molded plastic pivot pin bearing structure which exhibits acceptable wear, is more tolerant of elastomer formulations for the seal ring, and is inexpensive and relatively easy to fabricate to the precise dimensional tolerances required in a disposable plasmapheresis filtration cartridge.